1. Field of the Invention
Embodiments of the present invention relate generally to couplings between equipment and, more specifically, to a coupling or interface between a rotor and a balancing test machine.
2. Description of the Related Art
Turbo machines (also sometimes called “turbo rotating machines”) are a class of machines which include compressors, turbine engines, and the like, and which include rotors that, in operation, rotate at very high speeds, e.g., thousands or tens of thousands of revolutions per minute (RPMs). The rotor typically includes a shaft that is supported axially and radially for rotation in bearings. Given the size and weight of such rotors, even a small unbalance in a rotor can greatly reduce the number of operational hours for a turbo machine. For example, for a rotor having a weight of 500 pounds and an unbalance (e.g., center of gravity offset) of a mere 0.001 inch, the force resulting from the unbalance would be about 2000 pounds when the rotor is rotated at 12,000 RPM, which force is observed as vibrations that can rapidly ruin the bearings.
One way to address this problem is to balance test the rotors either as they are being assembled in stages or after they are completely assembled, and then to make adjustments to compensate for any detected unbalance. Such balance tests may be performed by connecting the rotors or rotor stages to a balancing test equipment which rotates the rotor under vacuum at high speed and has sensors, which detect imbalances, e.g., center of gravity offsets, during rotation.
A generalized high speed balancing test configuration is shown in FIG. 1. Inside a vacuum chamber 2, the pedestals 4 support a rotor 8 and a motor 6 of the balance testing machine is connected to the rotor 8 to be tested via a coupling or interface 9. In addition to a shaft, the rotor 8 may also have one or more elements coupled to the shaft, e.g., one or more impellers as described below. The coupling 9 transfers torque from the motor 6 to the rotor 8 and is provided as an element in the test system since there are typically many different sizes and configurations of rotors 8 to be balance tested by the balance testing machine and, therefore, the coupling 9 operates as an adapter between the various rotors 8 to be tested and the balance testing machine. An exemplary coupling 9 is shown in FIG. 2. Therein, it can be seen that the coupling 9 has a generally conical shape which tapers toward the end which fits onto the rotor 8, and has a relatively large diameter relative to the rotor 8. In practice, the coupling 9 is heat shrunk onto the rotor 8 prior to balance testing, and then removed for assembly into its respective turbo machine.
The use of such a coupling 9 as part of the balance testing process brings with it a number of drawbacks. The coupling 9 is relatively heavy, e.g., on the order of 20-30 kg, so that any eccentricity which it possesses makes the resulting unbalance that it adds to the testing system large enough to adversely affect balance testing, thereby potentially resulting in an unbalanced rotor 8. In fact, in some cases, the magnitude of the unbalance added by the coupling 9 may reach 200% of the acceptable unbalance tolerance for the rotor 8. Additionally, the method of attaching the coupling 9 to the rotor, i.e., heat shrinking, is time consuming, complex and may damage the rotor surface itself.
Accordingly, it would be desirable to design and provide a coupling for a rotor to a balance testing machine which overcomes the aforementioned drawbacks of existing couplings.